Frame structures have been proposed that are flexible in terms of the use of differing numerologies. A numerology is defined as the set of physical layer parameters of the air interface that are used to communicate a particular signal. A numerology is described in terms of at least subcarrier spacing and OFDM symbol duration, and may also be defined by other parameters such as fast Fourier transform (FFT)/inverse FFT (IFFT) length, transmission time slot length, and cyclic prefix (CP) length or duration. In some implementations, the definition of the numerology may also include which one of several candidate waveforms is used to communicate the signal. Possible waveform candidates may include, but are not limited to, one or more orthogonal or non-orthogonal waveforms selected from the following: Orthogonal Frequency Division Multiplexing (OFDM), Filtered OFDM (f-OFDM), Filter Bank Multicarrier (FBMC), Universal Filtered Multicarrier (UFMC), Generalized Frequency Division Multiplexing (GFDM), Single Carrier Frequency Division Multiple Access (SC-FDMA), Low Density Signature Multicarrier Code Division Multiple Access (LDS-MC-CDMA), Wavelet Packet Modulation (WPM), Faster Than Nyquist (FTN) Waveform, low Peak to Average Power Ratio Waveform (low PAPR WF), Pattern Division Multiple Access (PDMA), Lattice Partition Multiple Access (LPMA), Resource Spread Multiple Access (RSMA), and Sparse Code Multiple Access (SCMA).
These numerologies may be scalable in the sense that subcarrier spacings of different numerologies are multiples of each other, and time slot lengths of different numerologies are also multiples of each other. Such a scalable design across multiple numerologies provides implementation benefits, for example scalable total OFDM symbol duration in a time division duplex (TDD) context.
Table 1 below shows the parameters associated with some example numerologies, in the four columns under “Frame structure”. Frames can be configured using one or a combination of the four scalable numerologies. For comparison purposes, in the right hand column of the table, the conventional fixed LTE numerology is shown. The first column is for a numerology with 60 kHz subcarrier spacing, which also has the shortest OFDM symbol duration because OFDM symbol duration varies inversely with subcarrier spacing. This may be suitable for ultra-low latency communications, such as Vehicle-to-Any (V2X) communications. The second column is for a numerology with 30 kHz subcarrier spacing. The third column is for a numerology with 15 kHz subcarrier spacing. This numerology may have the same or similar configuration as in LTE. This may be suitable for broadband services. The fourth column is for a numerology with 7.5 kHz spacing, which also has the longest OFDM symbol duration among the four numerologies. This may be useful for coverage enhancement and broadcasting. Additional uses for these numerologies will be or become apparent to persons of ordinary skill in the art. Of the four numerologies listed, those with 30 kHz and 60 kHz subcarrier spacings are more robust to Doppler spreading (fast moving conditions), because of the wider subcarrier spacing. It is further contemplated that different numerologies may use different values for other physical layer parameters, such as the same subcarrier spacing and different cyclic prefix lengths.
It is further contemplated that other subcarrier spacings may be used, such as higher or lower subcarrier spacings. As illustrated in the example above, the subcarrier spacing of each numerology (7.5 kHz, 15 kHz, 30 kHz, 60 kHz) can be a factor of 2n times the smallest subcarrier spacing, where n is an integer. Larger subcarrier spacings that are also related by a factor of 2n, such as 120 kHz, may also or alternatively be used. Smaller subcarrier spacings that are also related by a factor of 2n, such as 3.75 kHz, may also or alternatively be used. The symbol durations of the numerologies may also be related by a factor of 2n. Two or more numerologies that are related in this way are sometimes referred to as scalable numerologies.
In other examples, a more limited scalability may be implemented, in which two or more numerologies all have subcarrier spacings that are integer multiples of the smallest subcarrier spacing, without necessarily being related by a factor of 2n. Examples include 15 kHz, 30 kHz, 45 kHz, 60 kHz, 120 kHz subcarrier spacings.
In still other examples, non-scalable subcarrier spacings may be used, which are not all integer multiples of the smallest subcarrier spacing, such as 15 kHz, 20 kHz, 30 kHz, 60 kHz.
TABLE 1Example set of NumerologiesPara-BaselinemetersFrame structure(LTE)time slot0.125 ms0.25 ms0.5 ms1 msTTI = 1 msLengthSub-60 kHz30 kHz15 kHz7.5 kHz15 kHzcarrierspacingFFT size5121024204840962048Symbol16.67 μs33.33 μs66.67 μs133.33 μs66.67 μsduration
In the example set of numerologies shown in Table 1, different cyclic prefix lengths can further be used in different numerologies having the same subcarrier spacing.
It should be understood that the specific numerologies of the examples of Table 1 are for illustration purposes, and that a flexible frame structure combining other numerologies can alternatively be employed.
OFDM-based signals can be employed to transmit a signal in which multiple numerologies coexist simultaneously. More specifically, multiple sub-band OFDM signals can be generated in parallel, each within a different sub-band, and each sub-band having a different subcarrier spacing (and more generally with a different numerology). The multiple sub-band signals are combined into a single signal for transmission, for example for downlink transmissions. Alternatively, the multiple sub-band signals may be transmitted from separate transmitters, for example for uplink transmissions from multiple electronic devices (EDs), which may be user equipments (UEs). In a specific example, filtered OFDM (f-OFDM) can be employed by using filtering to shape the frequency spectrum of each sub-band OFDM signal, thereby producing a frequency localized waveform, and then combining the sub-band OFDM signals for transmission. f-OFDM lowers out-of-band emission and improves transmission, and addresses the non-orthogonality introduced as a result of the use of different subcarrier spacings. Alternatively, a different approach can be used to achieve a frequency localized waveform, such as windowed OFDM (W-OFDM).
The use of different numerologies can allow the coexistence of a diverse set of use cases having a wide range quality of service (QoS) requirements, such as different levels of latency or reliability tolerance, as well as different bandwidth or signaling overhead requirements. In one example, the base station can signal to the ED an index representing a selected numerology, or a single parameter (e.g., subcarrier spacing) of the selected numerology. The signaling can be done in a dynamic or a semi-static manner, for example in a control channel such as the physical downlink control channel (PDCCH) or in downlink control information (DCI). Based on this signaling, the ED may determine the parameters of the selected numerology from other information, such as a look-up table of candidate numerologies stored in memory.
Resource blocks for each numerology may be transmitted on a grid. The grids for various numerologies are nested, in the sense that for a first numerology with a subcarrier spacing X, and a second numerology with a subcarrier spacing 2X, each RB on the grid with subcarrier spacing 2X will be aligned with two RBs on the grid with subcarrier spacing X. For each numerology there is a set of usable RBs on the respective grid having a middle subcarrier with a middle subcarrier frequency. In general, due to the nested structure of the RB grids, the middle subcarrier frequencies of usable RB ranges of different numerologies may not be aligned with each other.
Accordingly, the middle subcarrier of the baseband OFDM signal of each numerology may need to be shifted appropriately with respect to other numerologies to comply with the nested grid. These shifts can be signalled to the UE with an associated overhead.